The efficiency of Silicon solar cells is
reaching the theoretical limits outlined by Schokley & Queisser in 1961.
Improvements are necessary to decrease the price-to-performance ratio to the
levels required for becoming competitive with fossil fuels. Multijunction
photovoltaic devices have already exceeded the Shockley-Queisser limit for
single junction solar cells, and have proven themselves to be the highest
efficiency photovoltaic devices. However, this increased efficiency has to be
quantified through quantum efficiency (QE) measurements, which determine the
photon to electron conversion ratio. Traditional methods for measuring QE are
inadequate for multijunction devices since the full device is constrained by
the current-limiting junction. Our approach allows us to characterize the
individual junctions separately by flooding the non-measured junctions with
photons at their respective bandgap energies and voltage biasing the remaining
junction to short circuit conditions. We then sweep through a range of photon
energies, and the output current of the device is proportional to the quantum
efficiency of the isolated junction. Our data suggests that current
measurements are accurate, but more data must be gathered to confirm the
setup´s reliability. Namely, our results must be compared to data from other
calibrated QE setups, such as those from the National Renewable Energy
Laboratory (NREL), to confirm the accuracy of the setup.